1. Field of the Invention
This invention relates to an improved lithography system and method. More specifically, this invention relates to a lithography system and method using catadioptric exposure optics that projects high precision images without image flip.
2. Background Art
Lithography is a process used to create features on the surface of substrates. Such substrates can include those used in the manufacture of flat panel displays, circuit boards, various integrated circuits, and the like. A frequently used substrate for such applications is a semiconductor wafer. While this description is written in terms of a semiconductor wafer for illustrative purposes, one skilled in the art would recognize that this description also applies to other types of substrates known to those skilled in the art. During lithography, a wafer, which is disposed on a wafer stage, is exposed to an image projected onto the surface of the wafer by exposure optics located within a lithography apparatus. The image refers to the original, or source, image being exposed. The projected image refers to the image which actually contacts the surface of the wafer. While exposure optics are used in the case of photolithography, a different type of exposure apparatus may be used depending on the particular application. For example, x-ray or photon lithographies each may require a different exposure apparatus, as is known to those skilled in the art. The particular example of photolithography is discussed here for illustrative purposes only.
The projected image produces changes in the characteristics of a layer, for example photoresist, deposited on the surface of the wafer. These changes correspond to the features projected onto the wafer during exposure. Subsequent to exposure, the layer can be etched to produce a patterned layer. The pattern corresponds to those features projected onto the wafer during exposure. This patterned layer is then used to remove exposed portions of underlying structural layers within the wafer, such as conductive, semiconductive, or insulative layers. This process is then repeated, together with other steps, until the desired features have been formed on the surface of the wafer.
Exposure optics comprise refractive and/or reflective elements, i.e., lenses and/or mirrors. Currently, most exposure optics used for commercial manufacturing consist only of lenses. However, the use of catadioptric (i.e., a combination of refractive and reflective elements) exposure optics is increasing. The use of refractive and reflective elements allows for a greater number of lithographic variables to be controlled during manufacturing. The use of mirrors, however, can lead to image flip problems.
Image flip occurs when an image is reflected off of a mirror. FIG. 1 shows an example of image flip. In this example, if one were to hold up plain English text to a mirror, one would notice that the text, viewed in the mirror, would appear to be written backwards. Thus, an image of the letter “F” would be seen as “” in the mirror. This shows that when an image is reflected off of a mirror, the projected image results in an incorrect image orientation, i.e., the image transfer produces image flip. Of course, if the image is reflected off of two mirrors, the image orientation of the projected image would be correct because the image is flipped twice. Thus, an image of the letter “F” would be seen as “F” after the second reflection. Therefore, it can be seen that image flip results when an image is reflected an odd number of times. Conversely, it can be seen that image flip does not result when the image is reflected an even number of times.
Current lithographic systems typically include a reticle stage that is parallel to a wafer stage, such that the image from the reticle stage is projected downward onto the wafer stage. In addition, current lithographic systems typically include catadioptric exposure optics that require a magnifying mirror, such as a concave asphere. This mirror enhances the projected image and enables better exposure of the wafer. The parallel wafer and reticle stages together with the geometry of a magnifying mirror, however, makes it difficult for the catadioptric exposure optics to perform an even number of reflections.
To illustrate this point, FIG. 2 shows a simplified example lithographic system 200. System 200 shows a parallel reticle stage 202 and wafer stage 204 using catadioptric exposure optics 212, having a first mirror 206, a beam splitter 208, a quarter wave plate 209, and a magnifying mirror element group 210. In this example system 200, an image is projected from reticle stage 202 using P polarized light. This polarized light is reflected by first mirror 206 directly into magnifying mirror element group 210. It should be noted that quarter wave plate 209 can rotate the polarization angle of the light. The reflected image from first mirror 206 passes through beam splitter 208. This is due to the P polarization of the light being transmitted by beam splitter 208. The reflected image from magnifying mirror element group 210 has its polarization angle rotated 90°. This light is reflected at the beam splitter surface onto wafer 204. Thus, S polarization is not transmitted by beam splitter 208. Subsequently, the image is reflected directly out of magnifying mirror element group 210 that contains quarterwave plate 209. Besides flipping the image, magnifying mirror element group 210 also reverses the polarization of the image. Thus, the image reflected out of magnifying mirror element group 210 is then reflected by beam splitter 208, since the image now has the opposite polarization as beam splitter 208. The image is then projected onto parallel wafer stage 204. Using this configuration, an odd number of reflections occur. As a result, image flip problems occur.
Several alternative lithographic system designs, however, have attempted to overcome the image flip obstacle. One such design is a centrally obscured optical system design. FIG. 3 shows an example lithographic system 300 with a centrally obscured optical system design. System 300 shows a parallel reticle stage 302 and wafer stage 304 using catadioptric exposure optics 312 with a first mirror 306 and a magnifying mirror 308. In this example system 300, an image is projected from reticle stage 302 directly into magnifying mirror 308. It should be noted that the image projected from reticle stage 302 passes through first mirror 306. This is because first mirror 306 is polarized (in the same way as beam splitter 208 above). The image is then reflected directly out of magnifying mirror 308 and onto first mirror 306. Besides flipping the image, magnifying mirror 308 also reverses the polarization of the image. The image is then reflected downwards by first mirror 306, through a small hole 310 in magnifying mirror 308 and onto wafer stage 304. In this configuration, magnifying mirror 308 is in the path of the projected reflection of first mirror 306, which is why small hole 310 exists within magnifying mirror 308. The projected reflection of first mirror 306 travels through small hole 310 in magnifying mirror 308 to reach wafer stage 304. Using this configuration, an even number of reflections occur. Thus, there is no image flip problem. However, this configuration has its drawbacks. As the image is reflected by magnifying mirror 308, some of the image information (namely the portion of the image that passes through small hole 310 in magnifying mirror 308) is lost. This can produce aberrations or inconsistencies in the projected image.
Another lithographic system that has attempted to overcome the image flip obstacle is an off-axis design. FIG. 4 shows an example lithographic system 400 with an off-axis design. System 400 shows a parallel reticle stage 402 and wafer stage 404 using catadioptric exposure optics 412 with a first mirror 406 and a magnifying mirror 408. In this example system 400, an image is projected from reticle stage 402 onto a first mirror 406, reflected from first mirror 406 and into magnifying mirror 408, reflected out of magnifying mirror 408 and onto wafer stage 404. In this configuration, reticle stage 402 is off-axis from wafer stage 404. This is because the image is reflected away from the reticle stage in order to magnify it using magnifying mirror 408. As shown, there is a small angle 410 between first mirror 406 and wafer stage 404. Using this configuration, an even number of reflections occur. However, this configuration has its drawbacks. Magnifying mirror 408 does not directly (i.e., perpendicularly) receive the reflected image from first mirror 406. This is because magnifying mirror 408 must be able to receive a reflected image from first mirror 406 and reflect that image through a small angle 410 onto wafer stage 404. Further, magnifying mirror 408 does not directly reflect the image onto wafer stage 404. As a result, aberrations and perspective warping of the image can occur.
Therefore, it is difficult to create a lithographic system with catadioptric exposure optics that can produce a high quality image without image flip. Consequently, most lithographic systems today use a design similar to the design of FIG. 1. This design performs an odd-number of reflections that result in image flip problems. As a result, when exposing an image using these catadioptric exposure optics, it must be kept in mind that the projected image is the reverse of the desired image. This can lead to increased processing time and preparation. This problem is further compounded by the fact that most lithographic systems used today do not result in image flip. As a result, manufacturers that use both catadioptric exposure optics and non-catadioptric exposure optics (i.e., systems that have the image flip problem and systems that do not have the image flip problem) must use two reticle plates-one with each image orientation. This can lead to higher production costs.
In view of the above, what is needed is a lithographic system and method, using catadioptric exposure optics, which produces a high precision image without image flip.